1. Field of the Invention
This invention relates to the field of optical scanners, particularly those based on a micro-electromechanical (MEM) optical resonator.
2. Description of the Related Art
Optical scanning systems, i.e., systems which generate a scanning laser or light beam and detect the beam's reflection off of a target, find widespread use in such diverse fields as laser imaging, factory automation and information handling. Such a system typically includes a laser light source, an oscillating mirror, and a photodetector. The laser light is directed onto the oscillating mirror and the reflected beam traces a "scan line" each time the mirror moves from one oscillation extreme to the other. The scan line is in turn directed at a target such as a bar code, and laser light reflected from the target is detected by a photodetector which produces an output that provides information about the target. Two-dimensional (2-D) scanners, which can trace scan lines in two typically orthogonal directions, are also in common use.
A key component of an optical scanning system is the oscillating mirror. The frequency of oscillation determines the system's "scan rate", i.e., the number of times per second that a scan line is traced. A high scan rate enables a target to be scanned more quickly than it would be with a lower scan rate. The distance the mirror moves when oscillating determines the system's mechanical and optical "scan angles". Mechanical scan angle is defined as the angle between the plane of the mirror at one oscillation extreme and the other extreme. Optical scan angle is defined as the angle between a light beam reflected off the mirror at one oscillation extreme and the other extreme, which is equal to twice the mechanical scan angle. As used below, "scan angle" refers to the optical scan angle.
A high scan angle is critical to a scanning system's "resolution", i.e., the size of the smallest target detail which can be resolved by a scanning system. A system's resolution is determined by a variety of factors, including the quality of its light source, the optical components used, the scan angle and the distance from the light source to the target. If a target is far enough away from the source, even a system with a small scan angle produces a wide scan line which can trace a wide target. However, the spot size of a laser beam increases as it travels from its source, so that a large separation between source and target adversely affects resolution. A system having a wide scan angle offers the advantage of tracing a wide scan line while keeping the distance between source and target small, increasing system resolution.
Present optical scanners are generally based on galvanometric or oscillatory systems. A galvanometric scanner is a form of electric motor in which the armature is made to alternate between clockwise and counter-clockwise rotations of less than 360 degrees, typically producing a scan angle of less than 60 degrees. A plane mirror with its surface parallel to the axis of rotation is rigidly mounted on the projecting shaft of the armature. Galvanometric scanners tend to be bulky, heavy, expensive and have poor reliability. They are discussed in more detail in G. F. Marshall and J. Montagu, "Advances in Oscillatory Optical Scanners", SPIE Vol. 2383, pp. 440-448 (1995).
Oscillatory scanner systems operate by providing a stimulus necessary to cause a mirror and its mechanical mount to oscillate at the structure's mechanical resonance frequency, at which the system's scan angle is maximized. One such system, described in H. Goto and K. Imanaka, "Super compact dual axis optical scanning unit applying a torsional spring resonator driven by a piezoelectric actuator", SPIE Vol. 1544, Miniature and Micro-Optics: Fabrication and System Applications, pp. 272-280 (1991), uses a piezoelectric actuator attached to a resonator structure using an epoxy resin to excite the structure at its resonant frequency. The resonator structure includes mirror, torsional spring and inertia generating segments. This system has several shortcomings, however. The scan rate of the system is limited to about 288 Hz and the scan angle is limited to about 20 degrees. The piezoelectric actuator is bulky and requires a high driving voltage, and the device requires the separate manufacture and subsequent joining of the actuator and resonator components, resulting in a somewhat fragile hybrid device.
Another optical scanning system, described in U.S. Pat. No. 5,579,148 to Nishikawa et al., employs a resonator which includes four bimorph cells and three torsional springs surrounding a mirror. This multiplicity of components is hand-assembled into a bulky, complex, fragile structure which produces a scan angle of up to .+-.30 degrees, but requires a driving voltage of up to 20 V.sub.p-p.